1. Field of the Invention
The present invention relates to a crystalline ceramic fiber and a method of manufacturing such a crystalline ceramic fiber, and more particularly to a ceramic fiber suitable for use as a reinforcing material for a formed body of non-oxide ceramic such as silicon nitride or the like and a method of manufacturing such a ceramic fiber.
2. Description of the Prior Art
Sintered ceramic bodies are generally manufactured by forming ceramic powder into a certain shape, known as a green body, and then firing the green body. Since a ceramic material cannot easily be shaped once it is fired, it is formed into a desired shape before it is fired.
A formed ceramic body may be produced by any of various known processes. For example, a ceramic material may be pressed into a desired shape by a press or injection-molded into a desired shape by an injection molding machine. A ceramic slurry may be molded by a gypsum mold according to slip casting. Furthermore, a ceramic material may be pressed into a desired shape by a rubber press according to cold static hydraulic press. In order to produce a formed ceramic body having a small wall thickness or a complex shape, it is preferable to shape a ceramic slurry or a mixture of ceramic slurry and resin by slip casting, injection molding, or a process using a doctor blade.
In the slip casting process, a ceramic powder is dispersed in water, and the resultant slurry is put into a gypsum mold. The gypsum mold then absorbs and passes the water from the slurry, thereby producing a formed green body which is solidified.
According to the slip casting process, however, even if a binder or the like is mixed in such a ceramic powder, since the binding force between ceramic particles is not necessarily large, the mechanical strength of the formed body is not so large, and the formed body tends to be broken. When the ceramic slurry is dehydrated and dried in the gypsum mold, the formed body shrinks and hence is liable to crack. Therefore, it has been difficult to manufacture formed ceramic bodies of a small wall thickness or a complex shape without breakage or cracking.
In an attempt to solve the problem of breakage or cracking of formed ceramic bodies, it has been proposed to reinforce a formed ceramic body of silicon nitride with ceramic fibers that are produced by melting and spinning polysilazane (see, for example, Japanese laid-open patent publication No. 4-238874). It has been found, however, that a final sintered ceramic body reinforced with amorphous ceramic fibers produced by such a known process is apt to suffer defects therein.
Sintered ceramic bodies reinforced with crystalline ceramic fibers are prevented from becoming defective. However, when amorphous ceramic fibers produced by melting, spinning, and heating polysilazane are heat-treated simply in an inert atmosphere such as of nitrogen, as the fibers are crystallized, a number of whiskers are produced due to a vapor phase reaction, and surround the fibers, thus preventing the crystalline fibers from being extracted. Furthermore, before fibers of .alpha.- or .beta.-Si.sub.3 N.sub.4 suitable for reinforcing formed ceramic bodies of silicon nitride are formed, the decomposition of the ceramic fibers progresses to the extent that hollow fibers will finally be produced.